Symptom: deferred events on the Event Webhook, or SMTP replies 421 / 450.
1. Do not resend manually. Deferred messages are retried automatically for up to 72 hours.
2. Reduce volume to the deferring domain. More mail to a throttling server deepens the deferral.
3. Check the deferral rate per domain in your event data. One hot domain means a throttling policy; all domains means your IP or content reputation.
4. If you are on a new dedicated IP, this is expected during warmup: ISPs throttle unknown senders. Follow the warmup schedule instead of forcing volume.
5. Persistent 421s with good content and a warmed IP point at engagement: low opens and high complaints teach ISPs to throttle you. Fix the list and the content, not the retry logic.
4xx means try later, 5xx means stop. Never treat a 421 like a bounce.